The acetabulum is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint. There are three bones of the os coxae (hip bone) that come together to form the acetabulum. Contributing a little more than two-fifths of the structure is the ischium, which provides lower and side boundaries to the acetabulum. The ilium forms the upper boundary, providing a little less than two-fifths of the structure of the acetabulum. The rest is formed by the pubis, near the midline. It is bounded by a prominent uneven rim, which is thick and strong above, and serves for the attachment of the acetabular labrum, which reduces its opening, and deepens the surface for formation of the hip joint. At the lower part of the acetabulum is the acetabular notch, which is continuous with a circular depression, the acetabular fossa, at the bottom of the cavity of the acetabulum. The rest of the acetabulum is formed by a curved, crescent-moon shaped surface, the lunate surface, where the joint is made with the head of the femur. The acetabulum is also home to the acetabular notch, an attachment site for the ligamentum teres, a triangular, somewhat flattened band implanted by its apex into the antero-superior part of the fovea capitis femoris. The notch is converted into a foramen by the transverse acetabular ligament; through the foramen nutrient vessels and nerves enter the joint. This is what holds the head of the femur securely in the acetabulum.
The well-fitting surfaces of the femoral head and acetabulum, which face each other, are lined with a layer of slippery tissue called articular cartilage, which is lubricated by a thin film of synovial fluid. Friction inside a normal hip is less than one-tenth that of ice gliding on ice. Dislocation is the most common complication of hip replacement surgery. At surgery the femoral head is taken out of the socket, hip implants are placed and the hip put back into proper position. It takes eight to twelve weeks for the soft tissues injured or cut during surgery to heal. During this period, the hip ball can come out of the socket. The chance of this is diminished if less tissue is cut, if the tissue cut is repaired and if large diameter head balls are used. Doing the surgery from an anterior approach seems to lower dislocation rates when small diameter heads are used, but the benefit has not been shown when compared to modern posterior incisions with the use of larger diameter heads.
The acetabular cup is the component which is placed into the acetabulum (hip socket). Cartilage and bone are removed from the acetabulum and the acetabular cup is attached using friction or cement. Some acetabular cups are one piece, while others are modular. One piece (monobloc) shells are either UHMWPE (ultra-high-molecular-weight polyethylene) or metal, they have their articular surface machined on the inside surface of the cup and do not rely on a locking mechanism to hold a liner in place. A monobloc polyethylene cup is cemented in place while a metal cup is held in place by a metal coating on the outside of the cup. Modular cups consist of two pieces, a shell and liner. The shell is made of metal, the outside has a porous coating while the inside contains a locking mechanism designed to accept a liner. Two types of porous coating used to form a friction fit are sintered beads or a foam metal design to mimic the trabeculae of cancellous bone. Additional fixation is achieved as bone grows onto or into the porous coating. Screws can be used to lag the shell to the bone providing even more fixation. Polyethylene liners are placed into the shell and connected by a rim locking mechanism, ceramic and metal liners are attached with a Morse taper.
FIG. 1 depicts a conventional impactor, according to the background art. The exemplary impactor 100 is depicted as ready for impaction of a replacement acetabular cup 102. The acetabular cup 102 is temporarily attached to the cup engaging element 104 located at a first end 106 of the impactor body 108. The second end 110 is comprised of a handle 112 and an impaction anvil 114. The control elements of the impactor include a rotating outer end rod 120 with attached operating knob 122 and a rack 124 for changing the orientation of the rotating outer end rod 120 to maneuver the cup engaging element 104. Control lever 126 and crank 128 are additional control elements. Once the impactor 100 is aligned, the operator strikes the impactor anvil 114 with a hammer (not shown), driving the cup 102 into the hip and the cup 100 is securely seated.
Acetabular cup procedures require proper alignment for optimal outcome. Correct placement of the acetabular cup in total hip arthroplasty is a crucial step to achieve a satisfactory result and remains a challenge with free-hand techniques. Indeed, malpositioning can induce early loosening, high wear and postoperative dislocation. Various investigators have demonstrated that conventional free-hand positioning can result in a high percentage of unacceptable acetabular cup placements. Manual digitization can potentially cause measurement error which, in turn, can result in excessive tilt of the cup in the frontal plane. This is particularly problematic in obese patients where excess soft tissue can completely obscure bony landmarks.
Commercially available mechanical guides for alignment of the cup include a certain level of subjectivity on the part of the surgeon regarding alignment. Use of mechanical acetabular guides for intraoperative alignment leads to variations between the actual and desired implant orientation. These variations lead to a need for corrective procedures to revise the alignment of the implants.
It would be desirable to provide a system for alignment that relies on an objective system, minimizes subjectivity and thereby optimizes a beneficial outcome for the patient.